How monkey will survive as told by monkey head?

There are 21 monks. demon comes and tells he will kill them all. But will give a 50% chance of surviving. He makes them stand in a line one after the other and puts a black or white hat on each monk's head. A monk can't see which colour hat is on his head. the monster will start from the back of the line and ask each monk what coloured hat on his head. Monk can only say 'black or 'white'. If he answers correctly, he will survives else gets eaten alive. The head of the monks tells to give a day's time. Demon agrees and when he is gone, the head monk says that he might live or die tomorrow but he will ensure the rest will live. How?
Asked Nov 10, 2013

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